This invention relates generally to data communications, and more particularly to robust methods for packet acquisition in packet type data communications.
In packet type data communications, data is transferred from a data transmitter to a data receiver across a transmission medium in the form of variable or constant length data packets. The packets are ordered into frames to include a “payload” or data field, as well as a plurality of “header” or non-data fields which provide synchronizing and other housekeeping or identifying information useful for routing and processing the packets. Packet communication may be bursty in nature, with little or no signal activity occurring along the transmission medium during idle periods between packet transfers. The timing of packet transmissions is often irregular, and generally determined by data needs.
A well-known packet type data communication is the Ethernet system, one implementation of which is the 10 BASE-T system described in the IEEE standard 802.3. Different framing formats are used for different Ethernet implementations. An example of framing structure for an Ethernet system is given in Hioki, Telecommunications (1990 Prentice Hall 3rd ed.), at 430-433. The frame has a multiple byte preamble field, followed by destination address, source address, and data fields. Additional fields may include type fields, starting delimiters, length fields, and frame check sequence fields. In the described framing, the data field is between 46 and 1500 bytes in length, with total packet size between 72 bytes (576 bits) and 1526 bytes (12,208 bits) in length. A common preamble is used that comprises seven bytes of alternating 1s and 0s (starting with 1 and ending with 0). This particular preamble pattern produces a periodic waveform by the Manchester encoding circuit and allows the receiver to achieve frame synchronization with the packet.
Data is transmitted through coaxial cable, twisted wire pair or other transmission medium in packets using any of a variety of methods for coding data onto an analog medium, including amplitude modulation, frequency modulation and phase modulation. Two commonly used forms of phase modulation are binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). BPSK uses a two-phase modulation scheme—an in-phase signal and a 180 degree out-of-phase signal. During each baud (i.e., digital symbol transmission cycle), the transmitter sends one of the two signals. The phase sent determines the value of the bit transmitted (1 or 0). A single binary bit per baud is conveyed from transmitter to receiver during each baud time. QPSK uses a four phase modulation—an in-phase signal, a 180 degree out-of-phase signal, a ±90 degree phase signal, and a −90 degree phase signal. During each baud, the transmitter sends one of the four signals. Two binary bits per baud are thus conveyed during each baud time. See, e.g., U.S. Pat. No. 5,289,467. Another commonly used modulation method is quadrature amplitude modulation (QAM). QAM provides more bits per symbol transmission cycle by combining phase shift and amplitude keying to provide bit encoding within an in-phase and quadrature component (I-Q) modulated constellation space. A 16-bit QAM format, for example, uses 12 different phases and three different amplitudes to represent 16 possible carrier states, or four bits per baud cycle. See, Telecommunications, above, at 332-333.
Data is encapsulated or “packed” into frames at the transmitter, and decapsulated or “unpacked” from the frames at the receiver. The packet preamble provides a mechanism for establishing synchronization (“sync”) between the packing and unpacking operations. A preamble generator at the transmitter creates a preamble for each of the data packets and a preamble decoder at the receiver decodes the preamble of the data packet and determines sync for the packet. The preamble generated by the preamble generator may include a carrier detect interval, a carrier sync interval, a bit sync interval and a word sync pattern. The carrier detect interval is used by the receiver to identify the beginning of a packet. The carrier sync interval is used by a carrier synchronization circuit to identify the phase of the incoming transmission. The bit sync section of the preamble is used by a bit (baud) synchronization circuit to indicate the positions of baud symbols within the packet. The word sync pattern is used by the preamble decode to identify the beginning of a baud grouping, such as a forward error correcting (FEC) word. The remainder of the packet is transmitted and received as a collection of FEC words. See. e.g., U.S. Pat. No. 5,289,476.
The preamble functions as a synchronization symbol. To achieve frame synchronization or sync, each frame has a header with a predefined format and grouping of pseudo-random number sequences comprising symbols. The formatted signal including the preamble is transmitted to the receiver using the chosen modulation scheme. At the receiver, a correlator, utilizing a synchronization detection algorithm, is designed to match a predetermined symbol pattern with the received signal. Once the frame synchronization is established, the channel is characterized and the data symbols are recovered.
Noise is generated by the random motion of free electrons and molecular vibrations in all electronic components and conductors. The cumulative effect of all random noise generated internal and external to the data communication system, averaged over a period of time, is referred to as Gaussian noise, or additive white Gaussian noise (AWGN). (The noise is called “white” because it has frequencies distributed over the entire frequency spectrum, similar to the way white light includes all visible wavelengths of color.). See, e.g., Telecommunications, above, at 13-17 and 603.
Specific examples of packet data transmission include multipoint-to-point packet networks (such as CATV HFC upstream channels) and multipoint-to-multipoint packet networks (such as home phone wire networks). In both, a preamble (i.e. a sequence of a priori known symbols) is appended at the beginning of each data packet. Preamble symbols are usually of low constellations (such as BPSK or QPSK) while the payload (the data symbols) can be of higher constellations. The known preamble symbols allow for timing, amplitude and phase recovery and thereby acquisition of the data packets in the channel. The estimation error of these parameters in additive white Gaussian noise (AWGN) channels is inversely proportional to the length of the preamble and to the average power of the preamble symbols. Adequate estimation of these parameters is crucial for packet acquisition. When the estimation error is too high, the packet is lost.
The optimal estimator for AWGN channels is a correlator—a device that calculates the cross-correlation between the received signal and the preamble symbols. The estimate of the timing, amplitude and phase is determined according to the time, amplitude and phase of the peak of the signal at the correlator output. The use of matched filters correlators for synchronization and codeword identification is described in Stremler, Introduction to Communication Systems (1990 Addison-Wesley 3rd ed.), at 431-445. Both tapped delay line and time correlator realizations are illustrated.